Don’t Be Afraid of…Negative Feedback Douglas Self

Negative feedback is one of the most useful and omnipresent concepts in electronics. It can be used to control gain, to reduce distortion and improve frequency response, and to set input and output impedances, and one feedback connection can do all these things at the same time. Negative feedback comes in four basic modes, as in the four basic kinds of amplifier. It can be taken from the output in two different ways (voltage or current feedback) and applied to the amplifier input in two different ways (series or shunt). Hence there are four combinations.

However, unless you’re making something exotic like an audio constant-current source, the feedback is always taken as a voltage from the output, leaving us with just two feedback types, series and shunt, both of which are extensively used in audio. When series feedback is applied to a high-gain voltage amplifier, as in Figure 1.1(a), the following statements are true:

Negative feedback reduces voltage gain.

Negative feedback increases gain stability.

Negative feedback increases bandwidth.

Negative feedback increases amplifier input impedance.

Negative feedback reduces amplifier output impedance.

Negative feedback reduces distortion.

Negative feedback does not directly alter the signal-to-noise ratio.

If shunt feedback is applied to a voltage amplifier to make a transimpedance amplifier, as in Figure 1.1(c), all the above statements are still true, except since we have applied shunt rather than series negative feedback, the input impedance is reduced. The basic feedback relationship is Equation 1.1, which is dealt with at length in any number of textbooks, but it is of such fundamental importance that I feel obliged to include it here. The open-loop gain of the amplifier is A, and β is the feedback fraction, such that if in Figure 1.1(a) R1 is 2 kΩ and R2 is 1 kΩ, βis 1⁄3 . If A is very high, you don’t even need to know it; the 1 on the bottom becomes negligible, and the As on the top and bottom cancel out, leaving us with a gain of almost exactly 3.

Negative feedback can, however, do much more than stabilizing gain. Anything unwanted occurring in the amplifier, be it distortion or DC drift, or any of the other ills that electronics is to:

What negative feedback cannot do is improve the noise performance. When we apply feedback the gain drops, and the noise drops by the same factor, leaving the signal-to-noise ratio the same. Negative feedback and the way it reduces distortion is explained in much more detail in my book, Audio Power Amplifier Design Handbook

Douglas Selfstudied engineering at Cambridge University, then psychoacoustics at Sussex University. He has spent many years working at the top level of design in both the professional audio and hifi industries, and has taken out a number of patents in the field of audio technology. He currently acts as a consultant engineer in the field of audio design.

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